The packet transport networks were originally used for data traffics without the synchronization requirement; as networks evolve, however, the packet transport networks have to transmit not only data traffics but also Time Division Multiplexing (TDM) traffics, therefore the synchronization issue has to be solved for the packet transport networks.
In its Recommendation G.8261/Y.1361, International Telecommunication Union-Telecom (ITU-T) has defined some concepts and network configuration methods related to clock synchronization in packet transport networks, and also proposed that clock Synchronization Status Messages (SSMs) are defined and used with reference to Recommendation ITU-T G.707. But, the descriptions about the SSM in Recommendation G.707 are relatively simple, and are not comprehensive enough for protecting the clock link in complex networking scenarios. Based on the recommendation, the method that can provide protection for the clock link is that: each network element selects an available clock source that has the highest quality grade as a current clock source; if the current clock source is the Primary Reference Clock (PRC), then the quality grade of the clock is transmitted along all line directions; if the current clock source is the line clock, then the quality grade sent back along the direction should not be used for synchronization, and the quality grade of the current line clock is transmitted along other directions.
In G.707, the contents in the definition of the SSM are quite simple so that in complex networking scenarios, the quality grade of line clocks extracted by some nodes are not optimal, such as the shortest clock path; there are multiple reference clocks in the network, and in case of the failure of one clock, a clock loop may be formed.